In a CO2 gas-discharge laser, a lasing gas mixture within a laser-housing is energized by a radio-frequency (RF) discharge in the gas mixture. The discharge is struck between a pair of parallel spaced-apart electrodes. In a high-power CO2 laser, for example, a CO2 slab laser having an output power of 100 Watts (W) or more, the gas mixture typically includes CO2, nitrogen (N2), and helium (He), and is at a pressure between about 50 and 150 Torr. RF power for energizing the gas discharge is provided by the combined output of a plurality of RF power-amplifiers. These amplifiers are supplied by a single RF oscillator, the output of which is optionally pre-amplified.
In order to excite a gas discharge in a CO2 slab laser, an RF voltage of about 225 Volts (V) at a drive-frequency between about 80 and 100 megahertz (MHz) is required. Current in the discharge for a constant voltage applied to the electrodes increases linearly with power delivered into the discharge. The impedance of the discharge decreases as the RF power into the discharge is increased. A CO2 slab laser has an efficiency of about 10% for converting RF power into the discharge to laser-output power. By way of example, a CO2 laser having 250 W output requires about 2500 W of RF power at a current of about 11 Amps (A) to be delivered into the discharge. The impedance of the discharge is about 20 Ohms (Ω).
RF power-amplifiers are typically power transistor (MOSFET) modules, such as BLF278 modules available from Philips Corporation of Eindhoven, Holland. In order to provide 2500 W of RF power, a minimum of six BLF278 modules would be required. The outputs of the modules would need to be combined to form a single output that is provided to the discharge electrodes.
FIG. 1 schematically illustrates a prior-art RF-power combining arrangement 10 for driving a CO2 gas discharge laser. This arrangement is described in detail in U.S. Pat. No. 7,755,452, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. Only a brief description of the arrangement is presented here.
Arrangement 10 includes a stable RF oscillator 11, usually having a frequency between about 80 and 100 MHz as noted above. The oscillator output is pre-amplified by an amplifier 12. RF-output of the pre-amplifier is split by an N-way splitter 14, here, into three parts. Each part is connected to a corresponding power-amplifier 16. The amplifiers are designated PA1, PA2 and PA3. The choice of a three-way split with three power-amplifiers, here, is for convenience of description. The arrangement is effective with less than three power-amplifiers, or more than three power-amplifiers. Typically there would be an impedance-matching network between each splitter output and the corresponding power amplifier. These are not shown in FIG. 1 for convenience of illustration.
The number of parallel power amplifiers 16 is determined by the amount of RF power PL desired to be delivered to the load, divided by the power output rating of the power-amplifiers. The load, in this instance, is a discharge generated between a live or “hot” electrode 24 and a spaced-apart parallel ground electrode 26. The electrodes are located in a hermetically sealed laser housing 28 containing a lasing gas mixture, as discussed above. The load-impedance is determined, inter alia, by the length (L) and width (W) of the electrodes; the spacing (D) between the electrodes; and the pressure of the gas mixture. The impedance is higher when there is no lasing discharge struck between the electrodes.
The outputs of the power-amplifiers 16 are each connected to the inner conductor of a corresponding one of three co-axial transmission lines (cables) 20 having characteristic impedance Z0, with the outer conductor grounded at each end of the line as shown in FIG. 1. Strip line technology can be substituted for the co-axial cable for better compatibility with modern printed circuited board (PCB) technology. The center conductors of the cables are connected to a common node 21. The characteristic impedance at node 21 is Z0/3, i.e., Z0 divided by the number of amplifier outputs being combined. An impedance matching network (IMN) 22 is located between node 21 of power combiner 18 to match the impedance at the common node to the impedance of the laser discharge generated in the gap D between electrodes 24 and 26.
A problem that needs to be addressed in combining the outputs of multiple transistor power-amplifier modules is current-balancing and phase-adjustment of the outputs of each of the individual amplifiers. This is required in order to transfer maximum power into the load with maximum overall efficiency. If not efficiently combined, the transistor power amplifiers will experience additional losses which will manifest themselves as heat dissipation. Current balancing and phase adjustment is a tedious iterative procedure that is typically carried out manually. This current (amplitude) and phase balancing involves providing a variable reactance, such as a shunt variable capacitor, at the input of each of the power amplifiers to be balanced. This current and phase balancing procedure is complicated by cross-talk between the power-amplifier modules.
A current (amplitude) and phase-balancing method and apparatus that speeds the iterative process somewhat, by reducing the cross-talk problem, is described in U.S. patent application Ser. No. 13/216,091, filed Aug. 23, 2011, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference. The cross-talk problem is limited by providing switches or removable links, which allow current and phase balancing to be performed sequentially on an amplifier pair with others temporarily disconnected. The amplitude and phase of any amplifier is adjusted by means of the variable reactance shunt at the amplifier input as discussed above. The procedure is still an iterative, manual procedure but is somewhat shortened by the pair-wise execution. It would be advantageous, however, if at least one aspect of the current and phase-balancing could be made automatic.